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DS34C87T: RS-422 protection TVS diode junction capcitance

Stephen Thomas
Intellectual 710 points
Part Number: DS34C87T
Other Parts Discussed in Thread: DS34C86T,

Hi,

For a design implementation using DS34C87T and DS34C86T, we need unidirectional TVS diodes on the output and input lines for the protection of the device. We are interested to check the feasibility of using a TVS diode, SMBJ6.5A which has a junction capacitance which  @ 7.6V typ(Vbr) is approx 5000pF .Can this  capacitance add any problems like stress to device or degradation in terms of driver/receiver signal quality if we are using a data rate say typical 115,2k? 

Any information will be really helpful.

Thanks

Stephen

  • 0
    TI__Guru 69652 points
    Stephen,

    Looking at the VOH of the driver, it sits at 2.5V worst case outputting 20mA. Assuming a 5V VCC with a 10% drop internal to the device, 4.5V - 2.5V is a 2V drop to the output over 20mA, so we can say at worst case the output impedance is 100Ω. Combining that with worst case 5000pF from the TVS diode, the RC time constant is 500ns. Given a 115.2kHz data rate, that gives a period of 8.68us, and thus each bit will have a width of 4.34us. Given the 500ns RC time constant, and that bit width, you will see slight roll off on your rise time, but it should not degrade the signal enough to cause interpretation issues.

    Obviously the faster your data rate, the more likely you are to have communication errors, but at the rate you've chosen, that should be fine. However, if you add more TVS diodes, you will run into problems as well.

    Regards,